Valve Actuation Apparatus of Internal Combustion Engine and Rockable Cam for Use with the Same

ABSTRACT

In a valve actuation apparatus of an internal combustion engine employing a motion converter for converting a rotary motion of a drive cam into an oscillating motion of a rockable cam, the rockable cam has a cam contour surface, for operating an engine valve by the oscillating motion. The cam contour surface is contoured to have a valve-opening small lift area through which the cam contour surface extends from a base-circle area, a valve-opening middle lift area extending continuously from the valve-opening small lift area toward a cam-nose portion, and a valve-opening large lift area extending continuously from the valve-opening middle lift area toward a cam-nose top. A radius of curvature ρ 2  of a part of the valve-opening middle lift area, bordering the valve-opening small lift area, is set to be less than a radius of curvature ρ 3  of the valve-opening large lift area.

TECHNICAL FIELD

The present invention relates to a valve actuation apparatus of an internal combustion engine, and specifically to the improved technology of a rockable cam for use with the valve actuation apparatus.

BACKGROUND ART

In recent years, there have been proposed and developed various multinodular-link, rockable-cam operated valve actuation apparatus of internal combustion engines. One such valve actuation apparatus has been disclosed in Japanese Patent Provisional Publication No. 2002-256832 (hereinafter is referred to as “JP2002-256832”), corresponding to U.S. Pat. No. 6,550,437, and assigned to the assignee of the present invention. This type of valve actuation apparatus includes a variator (a multinodular-link motion converter), which can simultaneously vary both a valve lift and a working angle (a valve open period) of an intake valve. The variator is configured to convert rotary motion transmitted from an engine crankshaft into oscillating motion of the rockable cam for opening and closing the intake valve. By changing an initial attitude of the multinodular linkage of the variator, an initial sliding-contact point between the cam-contour surface of the rockable cam and the contact surface of a valve lifter can be changed for simultaneously varying both a valve lift and a working angle. Regarding the cam profile of the rockable cam disclosed in JP2002-256832, in particular, regarding the cam profile of the event area (the lift surface area) extending from the ramp surface area toward the cam-nose top, the positive acceleration area (of the event area), on the ramp-surface-area side, has a large radius of curvature, whereas the negative acceleration area (of the event area), on the cam-nose side, has a middle radius of curvature.

SUMMARY OF THE INVENTION

However, due to the specific cam profile of the event area of the rockable cam installed in the valve actuation apparatus disclosed in JP2002-256832, this apparatus exhibits a valve lift characteristic that an engine valve lift tends to increase substantially in proportion to an increase in working angle of the engine valve. Thus, the valve actuation apparatus of JP2002-256832 has the difficulty of ensuring a working angle suitable for an engine operating condition, while appropriately suppressing a valve lift, in particular, during middle and large valve-lift operating modes.

It is, therefore, in view of the previously-described disadvantages of the prior art, an object of the invention to provide a valve actuation apparatus of an internal combustion engine capable of ensuring a sufficient working angle suitable for an engine operating condition, while appropriately suppressing a valve lift.

In order to accomplish the aforementioned and other objects of the present invention, a valve actuation apparatus of an internal combustion engine comprises a drive cam adapted to be linked to a crankshaft of the engine in a manner so as to be driven by a transmitted torque from the crankshaft, a motion-transmission mechanism for converting a rotary motion of the drive cam into an oscillating motion, and a rockable cam configured to move with an oscillating motion in synchronism with the oscillating motion produced by the motion-transmission mechanism and having a curved cam contour surface, for opening and closing an engine valve by the oscillating motion of the rockable cam, wherein the cam contour surface of the rockable cam is contoured to have a valve-opening small lift area through which the cam contour surface extends from a base-circle area on which a lifter-crown contact surface rides when the engine valve is closed toward a cam-nose portion, a valve-opening middle lift area extending continuously from the valve-opening small lift area toward the cam-nose portion, and a valve-opening large lift area extending continuously from the valve-opening middle lift area toward a cam-nose top of the cam-nose portion, and wherein a radius of curvature of a part of the valve-opening middle lift area, bordering the valve-opening small lift area, is set to be less than a radius of curvature of the valve-opening large lift area.

According to another aspect of the invention, a valve actuation apparatus of an internal combustion engine comprises a drive cam adapted to be linked to a crankshaft of the engine in a manner so as to be driven by a transmitted torque from the crankshaft, a motion-transmission mechanism for converting a rotary motion of the drive cam into an oscillating motion, a rockable cam configured to move with an oscillating motion in synchronism with the oscillating motion produced by the motion-transmission mechanism and having a cam contour surface, for opening and closing an engine valve by the oscillating motion of the rockable cam, a variator configured to vary a valve-lift amount of the engine valve by changing an attitude of the motion-transmission mechanism and consequently by changing a state of the oscillating motion of the rockable cam, an actuator for driving the variator, and a controller configured to control the actuator depending on an operating condition of the engine, wherein the controller is configured to output a control signal to the actuator for bringing an operating characteristic of the engine valve closer to a maximum valve lift and maximum working angle characteristic via the variator during low-speed and low-load operation of the engine, wherein the cam contour surface of the rockable cam is contoured to have a valve-opening small lift surface through which the cam contour surface extends from a base-circle surface on which a lifter-crown contact surface rides when the engine valve is closed via a ramp surface toward a cam-nose portion, a valve-opening middle lift surface extending continuously from the valve-opening small lift surface toward the cam-nose portion, and a valve-opening large lift surface extending continuously from the valve-opening middle lift surface toward a cam-nose top of the cam-nose portion, and wherein a radius of curvature of a part of the valve-opening middle lift surface, bordering the valve-opening small lift surface, is set to be less than a radius of curvature of the valve-opening large lift surface, and the radius of curvature of the valve-opening large lift surface is set to be less than a radius of curvature of the valve-opening small lift surface.

According to a further aspect of the invention, for use with a valve actuation apparatus of an internal combustion engine, a rockable cam comprises a cam contour surface contoured to open and close an engine valve by an oscillating motion of the rockable cam, wherein the cam contour surface is contoured to have a valve-opening small lift area through which the cam contour surface extends from a base-circle area on which a lifter-crown contact surface rides when the engine valve is closed toward a cam-nose portion, a valve-opening middle lift area extending continuously from the valve-opening small lift area toward the cam-nose portion, and a valve-opening large lift area extending continuously from the valve-opening middle lift area toward a cam-nose top of the cam-nose portion, and wherein a radius of curvature of a part of the valve-opening middle lift area, bordering the valve-opening small lift area, is set to be less than a radius of curvature of the valve-opening large lift area.

The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating an embodiment of a valve actuation apparatus of an internal combustion engine, highlighting the essential part of the apparatus.

FIG. 2 is an enlarged side view of a rockable cam included in a multinodular-link motion converter of the apparatus of the embodiment.

FIG. 3 is a further enlarged view explaining the difference between the cam profile of the rockable cam of the multinodular-link motion converter of the apparatus of the embodiment and the cam profile of the rockable cam of a comparative example.

FIG. 4 is a characteristic diagram illustrating the relationship among a rockable-cam oscillating angle, a valve lift, an acceleration in valve movement (i.e., an acceleration in oscillating motion of the rockable cam), a radius of curvature, and a contact pressure, during oscillating motion of the rockable cam.

FIG. 5 is a partially enlarged characteristic diagram related to FIG. 4 and illustrating the relationship between a working angle and a valve lift.

FIG. 6A is a side view of the multinodular-link motion converter of the apparatus of the embodiment in partial cross-section taken in the direction of the arrow A of FIG. 1 during a valve closing period at a minimum valve-lift control mode, whereas FIG. 6B is a side view of the multinodular-link motion converter in partial cross-section taken in the direction of the arrow A of FIG. 1 during a valve opening period at the minimum valve-lift control mode.

FIG. 7A is a side view of the multinodular-link motion converter of the apparatus of the embodiment in partial cross-section taken in the direction of the arrow A of FIG. 1 during a valve closing period at a middle valve-lift control mode, whereas FIG. 7B is a side view of the multinodular-link motion converter in partial cross-section taken in the direction of the arrow A of FIG. 1 during a valve opening period at the middle valve-lift control mode.

FIG. 8A is a side view of the multinodular-link motion converter of the apparatus of the embodiment in partial cross-section taken in the direction of the arrow A of FIG. 1 during a valve closing period at a maximum valve-lift control mode, whereas FIG. 8B is a side view of the multinodular-link motion converter in partial cross-section taken in the direction of the arrow A of FIG. 1 during a valve opening period at the maximum valve-lift control mode.

FIG. 9A is a valve-lift characteristic diagram related to FIGS. 6A-6B during the minimum valve-lift control mode, FIG. 9B is a valve-lift characteristic diagram related to FIGS. 7A-7B during the middle valve-lift control mode, and FIG. 9C is a valve-lift characteristic diagram related to FIGS. 8A-8B during the maximum valve-lift control mode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, particularly to FIG. 1, the valve actuation apparatus of the embodiment is exemplified in a V-6 four-cycle internal combustion engine with an engine crankshaft and two cylinder banks having three pair of cylinders whose centerlines are set at a predetermined bank angle to each other, and applied to a multinodular-link, rockable-cam operated valve operating system on the intake valve side.

As shown in FIG. 1, the valve actuation apparatus of the embodiment is comprised of a pair of intake valves 2, 2, a variator (a motion converter) 4, a control mechanism 5, and a drive mechanism 6. Intake valves 2, 2 are engine valves, which are slidably installed on a cylinder head 1 via their valve guides, and permanently biased in a direction closing of the engine valves by respective valve springs 3, 3. Variator 4 is configured to simultaneously vary both a valve lift and a working angle for variably controlling an operating characteristic of each of intake valves 2, 2. Control mechanism 5 is provided for controlling an actuated position (or an initial attitude) of variator 4. Drive mechanism 6 is provided for driving the control mechanism 5.

Variator 4 is comprised of a cylindrical hollow drive shaft 7, a drive cam 8, a pair of rockable cams 10, 10 per cylinder, and a motion-transmission mechanism. Cylindrical hollow drive shaft 7 is rotatably supported by bearings in the upper part of cylinder head 1. Drive cam 8 is formed as an eccentric cam that is press-fitted or integrally connected onto the outer periphery of drive shaft 7. Rockable cams 10, 10 are oscillatingly or rockably supported on the outer periphery of drive shaft 7 and in sliding-contact with respective lifter-crown contact surfaces 9 a, 9 a of two valve lifters 9, 9, which are located at the valve stem ends of intake valves 2, 2, so as to operate the respective intake valves. In the shown embodiment, the motion-transmission mechanism is comprised of a multinodular linkage installed between the drive cam 8 and the rockable cam pair 10, 10, for converting rotary motion of drive cam 8 into oscillating motion of each of rockable cams 10, 10.

Drive shaft 7 is arranged in the fore-and-aft direction of the engine. Torque is transmitted from the engine crankshaft through a timing sprocket (not shown) fixedly connected to one axial end of drive shaft 7 via a timing chain (not shown) wound on the timing sprocket to drive shaft 7. As indicated by the arrow in FIG. 1, the direction of rotation of drive shaft 7 is set in a clockwise direction.

Drive cam 8 is shaped into a substantially ring shape. Drive cam 8 has an axial bore that is displaced from the geometric center of the ring-shaped drive cam 8. Drive cam 8 is fixedly connected to the outer periphery of drive shaft 7, so that the inner peripheral surface of the axial bore of drive cam 8 is press-fitted onto the outer periphery of drive shaft 7. Thus, the geometric center “Y” of drive cam 8 is offset from the shaft center “X” of the cylindrical hollow drive shaft 7 in the radial direction by a predetermined eccentricity.

Each of rockable cams 10, 10 is formed as a substantially raindrop-shaped cam. As described later in detail, rockable cams 10, 10 have the same cam profile. Rockable cams 10, 10 are formed integral with respective axial ends of a cylindrical hollow camshaft 11. Cylindrical hollow camshaft 11 is rotatably supported on drive shaft 7. The outer peripheral contacting surface of rockable cam 10, in sliding-contact with the upper contact surface 9 a of valve lifter 9, includes a cam contour surface 14. The base-circle portion of rockable cam 10 is integrally formed with or integrally connected to camshaft 11, to permit oscillating motion of rockable cam 10 on the axis of drive shaft 7. Of these rockable cams 10, 10, a cam-nose portion 12 (described later) of the first rockable cam 10 arranged closer to drive eccentric cam 8 than the second rockable cam 10, has a through hole, into which a connecting pin 20 (described later) fits, for mechanically linking the rockable cam pair 10, 10 to the lower end of a link rod (described later).

As best seen in FIGS. 2-3, the previously-discussed cam contour surface 14 is formed from a plurality of curved surfaces each having a different radius of curvature and continuous with each other. Concretely, cam contour surface 14 is mainly constructed by a base-circle surface 14 a (a base-circle area), a ramp surface 14 b (a ramp area), a valve-opening small lift surface 14 c corresponding to a positive acceleration area, a valve-opening middle lift surface 14 d corresponding to a short initial part of a negative acceleration area (in other words, a deceleration area) adjacent to the positive acceleration area, and a valve-opening large lift surface 14 e corresponding to the intermediate and last part of the negative acceleration area. Base-circle surface 14 a corresponds to the basal end of rockable cam 10 and contoured along the base circle of camshaft 11. Ramp surface 14 b is an impact-load cushioning cam-contour portion that avoids undue impact loading on the valve-train parts during operation of the valve actuation system. Valve-opening small lift surface 14 c is the positive acceleration area, extending continuously from the last part of ramp surface 14 b toward the cam-nose portion 12. Valve-opening middle lift surface 14 d is the short initial part of the negative acceleration area, extending continuously from the last part of valve-opening small lift surface 14 c toward the cam-nose portion 12. Valve-opening large lift surface 14 e is the intermediate and last part of the negative acceleration area, extending continuously from the last part of valve-opening middle lift surface 14 d toward the top of cam-nose portion 12. The detailed structure and configuration of cam contour surface 14 are further described later.

Base-circle surface 14 a, ramp surface 14 b, valve-opening small lift surface 14 c, valve-opening middle lift surface 14 c, and valve-opening large lift surface 14 e abut given positions of the lifter-crown contact surface 9 a of valve lifter 9, depending on the oscillatory position of rockable cam 10.

As seen from the side views of FIGS. 6A-6B, 7A-7B, and 8A-8B, the motion-transmission mechanism is comprised of a rocker arm 15 laid out above drive shaft 7, a link arm 16 mechanically linking one end (or a first arm portion 15 a) of rocker arm 15 to the drive cam 8, and a link rod 17 mechanically linking the other end (a second arm portion 15 b) of rocker arm 15 to the cam-nose portion 12 of rockable cam 10.

Rocker arm 15 is formed with an axially-extending center bore (a through opening). The rocker-arm center bore of rocker arm 15 is rotatably fitted onto the outer periphery of a control cam 22 (described later), to cause a pivotal motion (or an oscillating motion) of rocker arm 15 on the axis of control cam 22. The first arm portion 15 a of rocker arm 15 extends from the axial center bore portion in a first radial direction, whereas the second arm portion 15 b of rocker arm 15 extends from the axial center bore portion in a second radial direction substantially opposite to the first radial direction. The first arm portion 15 a of rocker arm 15 is rotatably pin-connected to link arm 16 by means of a connecting pin 18, while the second arm portion 15 b of rocker arm 15 is rotatably pin-connected to the upper end (a first end 17 a)of link rod 17 by means of a connecting pin 19.

Link arm 16 is comprised of a comparatively large-diameter annular base portion 16 a and a comparatively small-diameter protruding end portion 16 b radially outwardly extending from a predetermined portion of the outer periphery of large-diameter annular base portion 16 a. Large-diameter annular base portion 16 a is formed with a drive-cam retaining bore 16 c (see FIG. 6A), which is rotatably fitted onto the outer periphery of drive cam 8. On the other hand, small-diameter protruding end portion 16 b of link arm 16 is pin-connected to the first arm portion 15 a of rocker arm 15 by means of connecting pin 18.

Link rod 17 is formed into a substantially boomerang shape, as seen from the side view. The intermediate portion of link rod 17 has a substantially C-shaped lateral cross section, whereas each of the upper end 17 a and the lower end 17 b of link rod 17 is formed as two opposed flat plates. The upper link-rod end 17 a is pin-connected to the second arm portion 15 b of rocker arm 15 by means of connecting pin 19, whereas the link-rod lower end 17 b is pin-connected to the cam-nose portion 12 of rockable cam 10 by means of connecting pin 20.

Although it is not clearly shown in the drawings, a snap ring is fitted into a groove formed in the axial end of each of connecting pins 18-20, so as to prevent an undesirable connecting-pin drift, thus suppressing an undesirable axial displacement of each of link arm 16 and link rod 17.

Control mechanism 5 is a motion-converter attitude control mechanism that changes an initial actuated position (a fulcrum of oscillating motion of rocker arm 15) of the motion converter. As clearly shown in FIGS. 6A-6B, 7A-7B, and 8A-8B, control mechanism 5 includes a control shaft 21 and control cam 22. Control shaft 21 is located above drive shaft 7, and rotatably supported on the cylinder head 1 by means of the same bearing members as drive shaft 7. Control cam 22 is attached to the outer periphery of control shaft 21 and slidably fitted into and oscillatingly supported in a control-cam retaining bore formed in rocker arm 15. Control cam 22 serves as a fulcrum of oscillating motion of rocker arm 15.

Control shaft 21 is arranged in parallel with drive shaft 7 in such a manner as to extend in the longitudinal direction of the engine. Each of journal portions 21 a (see FIG. 1) of control shaft 21 is rotatably supported by a main bracket and a sub bracket, serving as split-type control-shaft journal bearings. Rotary motion of control shaft 21 in a normal-rotational direction or in a reverse-rotational direction is controlled via the drive mechanism 6.

Control cam 22 is integrally formed with control shaft 21, so that control cam 22 is fixed onto the outer periphery of control shaft 21. Control cam 22 is formed as an eccentric cam having a cylindrical cam profile. The axis (the geometric center) “P2” of control cam 22 is displaced a predetermined distance from the axis “P1” of control shaft 21 (see FIGS. 6A-6B).

Returning to FIG. 1, also provided is a stopper mechanism for restricting a maximum clockwise angular displacement and a maximum anticlockwise angular displacement of control shaft 21. The stopper mechanism is comprised of a stopper wall (not shown) protruded from the upper part of cylinder head 1 and a sector stopper member 23 fixedly connected to the outer periphery of control shaft 21.

As clearly shown in FIG. 1, drive mechanism 6 is comprised of a housing (not shown) fixedly connected to the rear end of cylinder head 1, a geared motor or an electric control-shaft actuator 24 fixed to one end of the housing, a ball-screw motion-transmitting mechanism (simply, a ball-screw mechanism) 25 installed in the housing so as to transmit a motor torque created by electric motor 24 to control shaft 21, and a return spring (a coiled spring) 26 (biasing means) for permanently biasing the control shaft 21 via the ball-screw mechanism 25 in a direction of rotation that the valve lift of each intake valve 2 is controlled to a minimum valve-lift amount. Return spring 26 is installed on the opposite side of the housing with respect to the installation position of electric motor 24.

Electric motor 24 is constructed by a proportional control type direct-current (DC) motor. Rotary motion of motor 24 (in the normal-rotational direction or in the reverse-rotational direction) is controlled in response to a control signal (a control current), which is generated from an electronic control unit (ECU) 27 (simply, a controller) and whose signal value is determined based on engine/vehicle operating conditions. Control unit 27 generally comprises a microcomputer. Control unit 27 includes an input/output interface circuitry (I/O), memories (RAM, ROM), and a microprocessor or a central processing unit (CPU). The input/output interface circuitry (I/O) of control unit 27 receives input information from various engine/vehicle sensors, namely a crank angle sensor, an airflow sensor, an engine temperature sensor (e.g., an engine coolant temperature sensor), a control-shaft angular position sensor, such as a potentiometer 34, and the like. The crank angle sensor is provided to detect an angular position (crankangle) of the engine crankshaft and engine speed (revolutions per minute). The engine temperature sensor is provided for sensing the actual operating temperature of the engine. The control-shaft angular position sensor (i.e., potentiometer 34) is provided to detect an actual angular position of control shaft 21. The airflow meter is provided for measuring or detecting a quantity of air flowing through an intake passage (an intake pipe), and consequently for detecting or estimating the magnitude of engine load. The processor of control unit 27 is configured to detect or estimate the current engine operating condition by feeding back sensor signals from the engine/vehicle sensors so as to output a control current determined based on the detected current engine operating condition to motor 24.

Ball-screw mechanism 25 is comprised of a ball-screw shaft (or a worm shaft) 28 coaxially aligned with and connected to the motor output shaft of motor 24, a substantially cylindrical, movable ball nut 29 threadably engaged with the outer periphery of ball-screw shaft 28, a link arm 30 fixedly connected to the rear end of control shaft 21, a link member 31 mechanically linking link arm 30 to ball nut 29, and recirculating balls interposed between the worm teeth of ball-screw shaft 28 and guide grooves cut in ball nut 29. Both ends of ball-screw shaft 28 are rotatably supported by ball bearings 32.

The right-hand end of ball-screw shaft 28 (viewing FIG. 1) and the motor output shaft of motor 24 are coaxially aligned with each other and connected to each other via a serrated member (not shown) formed with an internal serration (that is, by serration-connection), in a manner so as to transmit a motor torque created by motor 24 to ball-screw shaft 28, while permitting a slight axial movement of ball-screw shaft 28.

Ball nut 29 is formed into a substantially cylindrical shape. Ball nut 29 has spiral guide grooves cut in the inner peripheral wall surface of ball nut 29, for converting a rotary motion (input torque) of ball-screw shaft 28 into a rectilinear motion of ball nut 29 through the recirculating balls interposed between the worm teeth of ball-screw shaft 28 and guide grooves cut in ball nut 29. Link member 31 is pivotally pin-connected to a substantially intermediate portion of ball nut 29 by means of a pivot pin 33. In the engine stopped state, ball nut 29 is forced rightward (viewing FIG. 1), that is, toward motor 24 by the spring force of return spring 26, such that the valve lift of each intake valve 2 is controlled to a minimum valve-lift amount (i.e., a minimum working angle) with control shaft 21 biased toward its initial angular position (i.e., the spring-loaded position).

Link member 31 is formed into a substantially H-shape by mechanical pressing. The upper end of link member 31 is formed as two opposed flat plates installed to sandwich the intermediate portion of ball nut 29 therebetween and pin-connected to ball nut 29 by means of pivot pin 33. The lower end of link member 31 is also pin-connected to link arm 30 by means of a pivot pin (not shown).

As seen from the perspective view of FIG. 1, ball-screw mechanism 25 is configured to control the valve lift characteristic of each of intake valves 2, 2 to the minimum valve lift and minimum working angle characteristic via the control shaft 21, in a maximum rightward position of ball nut 29 (i.e., in a spring-loaded ball-nut position shown in FIG. 1), axially moved by rotary motion of ball-screw shaft 28 in one direction of rotation. Ball-screw mechanism 25 is also configured to control the valve lift characteristic of each of intake valves 2, 2 to the middle valve lift and middle working angle characteristic via the control shaft 21, in an intermediate position (viewing FIG. 1) of ball nut 29, axially moved leftward from the spring-loaded position by a predetermined displacement against the spring force of return spring 26 by rotary motion of ball-screw shaft 28 in the opposite rotational direction. Ball-screw mechanism 25 is also configured to control the valve lift characteristic of each of intake valves 2, 2 to the maximum valve lift and maximum working angle characteristic via the control shaft 21, in a maximum leftward position of ball nut 29, axially moved by rotary motion of ball-screw shaft 28 in the opposite rotational direction. As previously described, cam contour surface 14 of rockable cam 10 is mainly constructed by the base-circle surface 14 a (the base-circle area), the ramp surface 14 b (the ramp area), the valve-opening small lift surface 14 c corresponding to the positive acceleration area, the valve-opening middle lift surface 14 d corresponding to the short initial part of the negative acceleration area adjacent to the positive acceleration area, and the valve-opening large lift surface 14 e corresponding to the intermediate and last part of the negative acceleration area.

As appreciated from the enlarged views of FIGS. 2-3, regarding the radius of curvature of each of base-circle surface 14 a, ramp surface 14 b, and valve-opening small lift surface 14 c of the cam profile (see the cam profile indicated by the thick solid line in FIG. 3) of rockable cam 10 of the multinodular-link motion converter of the valve actuation apparatus of the embodiment, the radius of curvature of base-circle surface 14 a, the radius of curvature of ramp surface 14 b, and the radius of curvature ρ1 of valve-opening small lift surface 14 c are set or designed to be identical to those of the comparative example (i.e., the cam profile of the rockable cam of the multinodular-link motion converter of the valve actuation apparatus as disclosed in JP2002--256832). On the other hand, regarding the radius of curvature of each of valve-opening middle lift surface 14 d and valve-opening large lift surface 14 e of the cam profile (see the cam profile indicated by the thick solid line in FIG. 3) of rockable cam 10 of the multinodular-link motion converter of the valve actuation apparatus of the embodiment, the radius of curvature ρ2 of valve-opening middle lift surface 14 d and the radius of curvature ρ3 of valve-opening large lift surface 14 e are set or designed to be different from those of the comparative example (see the cam profile indicated by the one-dotted line in FIG. 3), i.e., the cam profile of the rockable cam of the multinodular-link motion converter of the valve actuation apparatus as disclosed in JP2002-256832.

Concretely, the radius of curvature ρ1 of valve-opening small lift surface 14 c (i.e., the valve-opening positive acceleration area) of the cam profile of the embodiment is formed into a gentle curve, which is a substantially straight line and has the largest radius of curvature in a similar manner to the valve-opening small lift surface of the cam profile of the comparative example. More concretely, regarding the cam profile of the shown embodiment, the radius of curvature ρ1 of valve-opening small lift surface 14 c is set to be greater than those of base-circle surface 14 a and ramp surface 14 b. On the other hand, the radius of curvature ρ2 of valve-opening middle lift surface 14 d of the cam profile of the embodiment is set to be less than that of the comparative example, and additionally the radius of curvature ρ3 of valve-opening large lift surface 14 e of the cam profile of the embodiment is set to be less than that of the comparative example. The radius of curvature of base-circle surface 14 a, the radius of curvature of ramp surface 14 b, the radius of curvature ρ1 of valve-opening small lift surface 14 c, the radius of curvature ρ2 of valve-opening middle lift surface 14 c, and the radius of curvature ρ3 of valve-opening large lift surface 14 e are collectively referred to as a “radius of curvature ρ” of the cam profile of rockable cam 10.

That is to say, as can be appreciated from comparison between the cam profiles of the embodiment (indicated by the thick solid line in FIG. 3) and the comparative example (indicated by the one-dotted line in FIG. 3), the radius of curvature ρ of the cam profile of rockable cam 10 of the embodiment and the radius of curvature of the cam profile of the rockable cam of the comparative example are identical to each other in each of the base-circle area, the ramp area, and the valve-opening positive acceleration area corresponding to valve-opening small lift surface 14 c. In contrast, the radius of curvature ρ2 of valve-opening middle lift surface 14 d, corresponding to the initial part of the valve-opening negative acceleration area adjacent to the valve-opening positive acceleration area, is set to be less than that of the comparative example. Also, the radius of curvature ρ3 of valve-opening large lift surface 14 e, corresponding to the intermediate and last part of the valve-opening negative acceleration area that extends continuously from the last part of valve-opening middle lift surface 14 d toward the top of cam-nose portion 12, is set to be greater than the radius of curvature ρ2 of valve-opening middle lift surface 14 d. Additionally, as a whole, the radius of curvature ρ3 of valve-opening large lift surface 14 e of the cam profile of the embodiment is set to be less than that of the comparative example.

By the way, there is one-to-one correspondence between (i) three different contact points (i.e., tangential lines or angular positions of rockable cam 10) between rockable-cam contour surface 14 and lifter-crown contact surface 9 a of valve lifter 9, respectively denoted by “a”, “b”, and “c” in FIG. 3 under a full lift attitude state and (ii) three different working angles “a”, “b”, and “c” in each of FIGS. 4-5, respectively corresponding to the rockable-cam oscillating angle θ achieved at the minimum lift control mode, the rockable-cam oscillating angle θ achieved at the middle lift control mode, and the rockable-cam oscillating angle θ achieved at the maximum lift control mode.

Briefly speaking, when comparing the cam profile of rockable cam 10 of the embodiment with the cam profile of the rockable cam of the comparative example, as seen from the valve-lift characteristic diagram of FIG. 9A (described later), at the operating mode of the minimum working angle D1, corresponding to the contact point (the tangential line) “a” of FIG. 3 between the valve-opening small lift surface 14 c (i.e., the valve-opening positive acceleration area) and the lifter-crown contact surface 9 a under a full lift attitude state, the valve lift characteristics of the embodiment and the comparative example tend to become identical to each other. As seen from the valve-lift characteristic diagram of FIG. 9B (described later), at the operating mode of the middle working angle D2, corresponding to the contact point (the tangential line) “b” of FIG. 3 between the valve-opening middle lift surface 14 d and the lifter-crown contact surface 9 a under a full lift attitude state, a peak lift L2 of the embodiment tends to be lowered by a lift difference α in comparison with a peak lift L2′ of the comparative example. As seen from the valve-lift characteristic diagram of FIG. 9C (described later), at the operating mode of the maximum working angle D3, corresponding to the contact point (the tangential line) “c” of FIG. 3 between the valve-opening maximum lift surface 14 e and the lifter-crown contact surface 9 a under a full lift attitude state, a peak lift L3 of the embodiment tends to be lowered by a large lift difference β in comparison with a peak lift L3′ of the comparative example.

The difference between the rockable-cam profiles of the embodiment and the comparative example, in particular, the cam-profile difference related to valve-opening middle lift surface 14 d and valve-opening large lift surface 14 e between the embodiment and the comparative example, is hereunder explained in detail in reference to FIGS. 6A-6B, 7A-7B, 8A-8B, and 9A-9C, from the viewpoint of the behavior of rockable cam 10 during operation of the variator 4 (the motion converter), in particular, in each of the valve-opening positive acceleration area and the valve-opening negative acceleration area.

Assuming that the previously-discussed rockable-cam oscillating angle θ of rockable cam 10 is defined as an angle between (i) the reference line “Q” passing through both the center “◯” of oscillating motion of rockable cam 10 and the cam-nose top of rockable cam 10 and oscillating together with rockable cam 10 and (ii) the perpendicular line “R” perpendicular to the lifter-crown contact surface 9 a, a valve lift (an opening-period valve-lift amount) y is defined as a distance between the lifter-crown contact surface 9 a and the base-circle surface 14 a (see FIG. 2), an acceleration (an valve-opening-period acceleration) y″ of rockable cam 10 (see FIG. 4) is defined as the second-order derivative d²y/dθ² (unit: mm/rad²) of valve lift y (unit: mm) with respect to rockable-cam oscillating angle θ (unit: rad).

From the viewpoint of characteristics of valve lift y and rockable-cam acceleration y″ (=d²y/dθ²), the profile of cam contour surface 14 is classified into five sections, namely, (i) the base-circle area (base-circle surface 14 a), (ii) the ramp area (ramp surface 14 b), (iii) the positive acceleration area (i.e., valve-opening small lift surface 14 c), (iv) the short initial part of the negative acceleration area (i.e., valve-opening middle lift surface 14 d), and (v) the intermediate and last part of the negative acceleration area (i.e., valve-opening large lift surface 14 e). In the base-circle area (base-circle surface 14 a), the amount of valve lift is zero, that is, y=0. The initial part of the ramp area (ramp surface 14 b), adjacent to base-circle surface 14 a, serves to introduce a small acceleration y″. Thereafter, the intermediate and last part of the opening ramp serves to return acceleration y″ to zero so as to stably increase the amount of valve lift y. Valve-opening small lift surface 14 c is included in the positive acceleration area of an event area (a lift surface area), in which acceleration y″ is positive. Valve-opening middle lift surface 14 d and valve-opening large lift surface 14 e are included in the negative acceleration area of the event area, in which acceleration y″ is negative.

The radius of curvature ρ of cam contour surface 14 of rockable cam 10, a load F acting between rockable cam 10 and lifter-crown contact surface 9 a, and a contact pressure P between cam contour surface 14 of rockable cam 10 and lifter-crown contact surface 9 a of valve lifter 9 are defined or represented by the following expressions.

ρ=Rc+y+y″=Rc+y+d ² y/dθ ²

F=F0+k×y

P={(E×F)/(2π×ρ×w)}^(1/2)

where Rc denotes a radius of curvature of the base-circle area (base-circle surface 14 a), F0 denotes a spring load of valve spring 3, k denotes a spring constant of valve spring 3, w denotes a width of rockable cam 10, and E denotes a cam-and-lifter equivalent Young's modulus of rockable cam 10 and valve lifter 9.

As can be seen from the characteristic diagram of FIG. 4, the radius of curvature ρ2 of valve-opening middle lift surface 14 c, immediately after the start of the negative acceleration area, in other words, at the contact point “b” of valve-opening middle lift surface 14 d with the lifter-crown contact surface 9 a (under a full lift attitude), corresponding to the middle working angle D2, is set or designed to the smallest radius of curvature. Owing to the smallest radius of curvature ρ2 at the contact point “b” of valve-opening middle lift surface 14 d shown in FIG. 2, the peak lift L2 at the middle working angle “b” (=D2) shown in FIG. 4 becomes an appropriately suppressed lift amount, and thus the load F acting between rockable cam 10 and lifter-crown contact surface 9 a also becomes suppressed. As a result, the contact pressure P between cam contour surface 14 of rockable cam 10 and lifter-crown contact surface 9 a of valve lifter 9 becomes suppressed immediately after the start of the negative acceleration area.

At the contact point “c” of valve-opening large lift surface 14 e with the lifter-crown contact surface 9 a (under a full lift attitude), corresponding to the maximum working angle D3, for the purpose of appropriately suppressing the peak lift L3 at the large working angle “c” (=D3), as seen from the characteristic diagram of FIG. 4, acceleration y″ is set to moderately reduce from the middle working angle “b” (=D2) to the large working angle “c” (=D3), while keeping the radius of curvature ρ substantially at the smallest value. However, in the negative acceleration area extending from the middle working angle “b” (=D2) to the large working angle “c” (=D3), due to an increase in valve-lift amount, the load F acting between rockable cam 10 and lifter-crown contact surface 9 a tends to become great, and thus the contact pressure P between cam contour surface 14 of rockable cam 10 and lifter-crown contact surface 9 a of valve lifter 9 also tends to increase. As a result of this, undesirable abrasion or wear between cam contour surface 14 of rockable cam 10 and lifter-crown contact surface 9 a of valve lifter 9 is apt to occur. For this reason, in the negative acceleration area extending from the middle working angle “b” (=D2) to the large working angle “c” (=D3), the radius of curvature ρ3 of valve-opening large lift surface 14 e is set to be greater than the smallest radius of curvature ρ2 of valve-opening middle lift surface 14 d in a manner so as to introduce an appropriately adjusted acceleration that suppresses an excessive increase in contact pressure P.

The operation of the valve actuation apparatus of the embodiment is hereinafter described in detail.

In the engine stopped state, ball nut 29 is forced rightward (viewing FIG. 1), that is, toward electric motor 24 by the spring force of return spring 26, and then the maximum angular displacement of control shaft 21 in one rotational direction (i.e., the maximum anticlockwise angular displacement, viewing FIGS. 6A-6B) is restricted by the stopper mechanism. As a result, the valve lift of each intake valve 2 is controlled to a minimum valve-lift amount (i.e., a minimum working angle).

By rotating electric motor 24 in a normal-rotational direction or in a reverse-rotational direction is controlled responsively to a control current, outputted from control unit 27 and determined based on a change in the engine operating condition after the engine has been started up, ball-screw shaft 28 is also rotated in the same rotational direction as the motor output shaft. As a result, a leftward or rightward displacement of ball nut 29 (viewing FIG. 1) occurs and thus rotary motion of control cam 22 (control shaft 21) in a normal-rotational direction or in a reverse-rotational direction occurs.

As shown in FIGS. 6A-6B, for instance, when the geometric center “P2” of control cam 22 revolves around the center “P1” of control shaft 21 with the maximum anticlockwise angular displacement of control shaft 21, the radially thick-walled portion of control cam 22 shifts to the upper right position with respect to drive shaft 7, with the result that the pivot (the connected point by connecting pin 19) between the second arm portion 15 b of rocker arm 15 and the first rod end 17 a of link rod 17 also shifts upward with respect to drive shaft 7. As a result, the cam-nose portion 12 of each of rockable cams 10, 10 is forcibly pulled up via the second rod end 17 b of link rod 17. As viewed from the front end of drive shaft 7, a change in the attitude of rockable-cam pair 10, 10 to the clockwise direction occurs, such that the angular position of each rockable cam 10 shown in FIGS. 6A-6B is relatively shifted to the clockwise direction with respect to the angular position of each rockable cam 10 shown in FIGS. 8A-8B.

With control cam 22 held at the angular position shown in FIGS. 6A-6B, when drive cam 8 is rotated, the first arm portion 15 a of rocker arm 15 is pushed up via link arm 16 and thus its valve-opening lift is transmitted via link rod 17 to rockable cams 10, 10 and respective lifters 9, 9. At this time, a valve-opening amount becomes sufficient small. Thus, the lift of each of intake valves 2, 2 becomes a small lift L1 and simultaneously the intake-valve working angle becomes a small working angle D1 (see the minimum valve lift L1 and minimum working angle D1 characteristic shown in FIG. 9A). As a result of this, intake valve open timing (IVO) of each of intake valves 2, 2 becomes phase-retarded and intake valve closure timing (IVC) of each of intake valves 2, 2 becomes phase-advanced.

As shown in FIGS. 7A-7B, when the geometric center “P2” of control cam 22 revolves in the opposite direction (clockwise, viewing FIGS. 7A-7B) around the center “P1” of control shaft 21 owing to a change from the previously-discussed engine operating condition to another engine operating condition, and thus control cam 22 is displaced to an intermediate angular position, the radially thick-walled portion of control cam 22 slightly downwardly shifts toward drive shaft 7, with the result that the pivot (the connected point by connecting pin 19) between the second arm portion 15 b of rocker arm 15 and the first rod end 17 a of link rod 17 also shifts slightly downward. As a result, the cam-nose portion 12 of each of rockable cams 10, 10 is forcibly slightly pushed down via the second rod end 17 b of link rod 17. As viewed from the front end of drive shaft 7 in FIGS. 7A-7B, the angular position of each rockable cam 10 is relatively shifted to the anticlockwise direction from the angular position of each rockable cam 10 shown in FIGS. 6A-6B.

With control cam 22 shifted to the intermediate angular position (see FIGS. 7A-7B) located in a substantially middle of the angular position shown in FIGS. 6A-6B and the angular position shown in FIGS. 8A-8B, when drive cam 8 is rotated, the first arm portion 15 a of rocker arm 15 is pushed up via link arm 16 and thus its valve-opening lift is transmitted via link rod 17 to rockable cams 10, 10 and respective lifters 9, 9. Due to a change in the attitude of the multinodular-linkage motion-transmission mechanism of variator 4, that is, due to an attitude change of rockable cam 10 from the angular position “a” to the angular position “b” under a full lift attitude state, the lift of each of intake valves 2, 2 becomes increased to a middle lift L2 and simultaneously the intake-valve working angle becomes increased to a middle working angle D2 (see the middle valve lift L2 and middle working angle D2 characteristic shown in FIG. 9B). As a result of this, intake valve open timing IVO of each of intake valves 2, 2 becomes phase-advanced and intake valve closure timing IVC of each of intake valves 2, 2 becomes phase-retarded, as compared to the minimum valve lift L1 and minimum working angle D1 characteristic shown in FIG. 9A.

As can be appreciated from the middle valve lift L2 and middle working angle D2 characteristic shown in FIG. 9B, note that, for the same middle working angle D2, created by each of the comparative example and the embodiment during the middle lift control mode, the peak lift L2 realized by rockable cam 10 of the embodiment (indicated by the thick solid line in FIG. 9B) tends to be lowered by a lift difference a in comparison with the peak lift L2′ of the comparative example (indicated by the one-dotted line in FIG. 9B).

After this, as shown in FIGS. 8A-8B, when the geometric center “P2” of control cam 22 further revolves around the center “P1” of control shaft 21 owing to a further engine-operating-condition change to another engine operating condition and then control cam 22 is displaced to its maximum clockwise angular position, the radially thick-walled portion of control cam 22 further downwardly shifts toward drive shaft 7, with the result that the pivot (the connected point by connecting pin 19) between the second arm portion 15 b of rocker arm 15 and the first rod end 17 a of link rod 17 further shifts downward. As a result, the cam-nose portion 12 of each of rockable cams 10, 10 is further pushed down via the second rod end 17 b of link rod 17. As viewed from the front end of drive shaft 7 in FIGS. 8A-8B, the angular position of each rockable cam 10 is further shifted to the anticlockwise direction from the angular position of each rockable cam 10 shown in FIGS. 7A-7B.

With control cam 22 shifted to the maximum clockwise angular position (see FIGS. 8A-8B), when drive cam 8 is rotated, the first arm portion 15 a of rocker arm 15 is pushed up via link arm 16 and thus its valve-opening lift is transmitted via link rod 17 to rockable cams 10, 10 and respective lifters 9, 9. At this time, a valve-opening amount becomes sufficient large. Thus, the lift of each of intake valves 2, 2 becomes a large lift L3 and simultaneously the intake-valve working angle becomes a large working angle D3 (see the maximum valve lift L3 and maximum working angle D3 characteristic shown in FIG. 9C). As a result of this, intake valve open timing IVO of each of intake valves 2, 2 becomes earliest and intake valve closure timing IVC of each of intake valves 2, 2 becomes latest, as compared to the middle valve lift L2 and middle working angle D2 characteristic shown in FIG. 9B.

As can be appreciated from the maximum valve lift L3 and maximum working angle D3 characteristic shown in FIG. 9C, the magnitude of working angle D3 itself, created by rockable cam 10 of the embodiment (indicated by the thick solid line in FIG. 9C), becomes identical to that created by the rockable cam of the comparative example (indicated by the one-dotted line in FIG. 9C) during the large lift control mode, but the peak lift L3 realized by rockable cam 10 of the embodiment (indicated by the thick solid line in FIG. 9C) tends to be lowered by a lift difference R in comparison with the peak lift L3′ of the comparative example (indicated by the one-dotted line in FIG. 9C).

That is, as seen from the characteristic diagram of FIG. 5 illustrating the relationship between valve-lift amount y and working angle, valve-lift amount y created by rockable cam 10 of the embodiment (indicated by the thick solid line in FIG. 5) and the valve-lift amount created by the rockable cam of the comparative example (indicated by the one-dotted line in FIG. 5) tend to change at almost the same gradient y in a working-angle range from the minimum working angle D1 (angular position “a”) to the middle working angle D2 (angular position “b”). In contrast, in a working angle range from the middle working angle D2 (angular position “b”) to the maximum working angle D3 (angular position “c”), the valve-lift characteristic curve of rockable cam 10 of the embodiment (indicated by the thick solid line in FIG. 5) tends to gradually deviate downwardly from that of the comparative example (indicated by the one-dotted line in FIG. 5), such that the valve-lift amount of the embodiment can be lowered than that of the comparative example.

As discussed above, according to the specific cam profile of rockable cam 10 of the valve actuation apparatus of the embodiment, as appreciated from the lift characteristic diagrams of FIGS. 9B-9C, the middle valve-lift amount L2 (the top lift at the middle lift control mode) and the maximum valve-lift amount L3 (the top lift at the maximum lift control mode) of the embodiment (see the lift characteristic curves indicated by the thick solid line in FIGS. 4-5) can be suppressed lower than those of the comparative example (see the lift characteristic curves indicated by the one-dotted line in FIGS. 4-5). The reason why the valve-lift amount of the embodiment can be appropriately suppressed in comparison with the comparative example in the middle-and-large working angle range is that, regarding rockable-cam contour surface 14 classified into the five sections, the radius of curvature ρ2 of valve-opening middle lift surface 14 d immediately after the start of the negative acceleration area, in other words, the radius of curvature ρ2 at the angular position “b” (corresponding to the middle working angle D2), is set or designed to be less than that of the comparative example, while maintaining and realizing almost the same positive acceleration area (i.e., valve-opening small lift surface 14 c of the largest radius of curvature ρ1). Additionally, the reason for this is that the radius of curvature ρ3 of valve-opening large lift surface 14 e, corresponding to the intermediate and last part of the negative acceleration area, is set or designed to be less than that of the comparative example.

During low-speed and low-load operation, involving idling operation at low revolution speeds, after the engine has been started up, electric motor 24 is driven in a rotational direction responsively to a control current generated from control unit 27 and thus ball-screw shaft 28 is rotated in the same rotational direction as the motor output shaft via recirculating balls in rolling-contact between the worm teeth of ball-screw shaft 28 and guide grooves cut in ball nut 29, thereby producing a maximum rectilinear motion of ball nut 29 in the ball-nut axial leftward direction (viewing FIG. 1) that ball nut 29 moves apart from electric motor 24. By this, control cam 22 is displaced to the maximum clockwise angular position (see FIGS. 8A-8B) by revolving motion of the geometric center “P2” of control cam 22 around the center “P1” of control shaft 21, and thus the cam-nose portion 12 of each of rockable cams 10, 10 is forcibly pushed down via the second rod end 17 b of link rod 17. As a result, a maximum attitude change of each rockable cam 10 to the anticlockwise direction occurs. With control cam 22 shifted to the maximum clockwise angular position (see FIGS. 8A-8B), when drive cam 8 is rotated, the first arm portion 15 a of rocker arm 15 is pushed up via link arm 16 and thus its valve-opening lift is transmitted via link rod 17 to rockable cams 10, 10 and respective lifters 9, 9. Hence, a valve-opening amount becomes sufficient large. Thus, the lift of each of intake valves 2, 2 becomes a large lift L3 and simultaneously the intake-valve working angle becomes a large working angle D3 (see the maximum valve lift L3 and maximum working angle D3 characteristic shown in FIG. 9C). As a result of this, intake valve open timing IVO of each of intake valves 2, 2 becomes earliest and intake valve closure timing IVC of each of intake valves 2, 2 becomes latest, as compared to the middle valve lift L2 and middle working angle D2 characteristic shown in FIG. 9B.

As already described, as clearly shown in FIGS. 9B-9C, each of working angles D2-D3, created by rockable cam 10 of the embodiment during the middle and large lift control modes, becomes almost the same magnitude as each of middle and large working angles, created by the rockable cam of the comparative example during the middle and large lift control modes. On the other hand, regarding the valve-lift amount created during the middle lift control mode, the middle valve-lift amount L2 (the peak lift) of the embodiment (indicated by the thick solid line in FIG. 9B) tends to be lowered by a lift difference a in comparison with the middle valve-lift amount L2′ (the peak lift) of the comparative example (indicated by the one-dotted line in FIG. 9B). Also, regarding the valve-lift amount created during the large lift control mode, the large valve-lift amount L3 (the peak lift) of the embodiment (indicated by the thick solid line in FIG. 9C) tends to be lowered by a lift difference β in comparison with the large valve-lift amount L3′ (the peak lift) of the comparative example (indicated by the one-dotted line in FIG. 9C).

As discussed above, during low-speed and low-load operation, involving idling operation at low revolution speeds, according to the cam profile of cam contour surface 14 of rockable cam 10 of the embodiment, it is possible to ensure the maximum working angle D3 having the same magnitude as the maximum working angle created by the rockable cam of the comparative example, while suppressing the maximum valve-lift amount L3 lower than that created by the rockable cam of the comparative example. This contributes the reduced contact pressure P between cam contour surface 14 of rockable cam 10 and lifter-crown contact surface 9 a of valve lifter 9 and stable combustion during low-speed and low-load operation, involving idling operation. As a result, friction between cam contour surface 14 and lifter-crown contact surface 9 a can be sufficiently reduced, thus suppressing abrasion or wear between cam contour surface 14 and lifter-crown contact surface 9 a from occurring. Also, it is possible to ensure stable engine revolution speeds as well as improved fuel economy during low-speed and low-load operation of the internal combustion engine.

By the way, there are two methods to realize a so-called “Atkinson cycle” that an expansion ratio of an internal combustion engine is set to be greater than a compression ratio to improve a thermal efficiency (a combustion efficiency), namely, (i) one being a so-called early intake-valve-closing combustion cycle at which intake valve closure timing IVC is controlled to an earlier timing value, for instance approximately 90 degrees of crankangle BBDC (before the piston bottom dead center) on the intake stroke and (ii) the other being a so-called late intake-valve-closing combustion cycle at which intake valve closure timing IVC is controlled to a later timing value, for instance approximately 90 degrees of crankangle ABDC (after the piston bottom dead center) on the intake stroke. Hereupon, the previously-noted compression ratio determined by intake valve closure timing IVC means an effective compression ratio, often denoted by Greek letter “ε′”, which is generally defined as a ratio of the effective cylinder volume corresponding to the maximum working medium volume to the effective clearance volume corresponding to the minimum working medium volume. Note that the effective compression ratio “ε′” is thermodynamically distinguished from a geometrical or mechanical compression ratio, often denoted by Greek letter “ε”, which is generally defined as a ratio (V1+V2)/V1 of full volume (V1+V2) existing within the engine cylinder and combustion chamber with the piston at BDC (bottom dead center) to the clearance-space volume (V1) with the piston at TDC (top dead center). In contrast, in a reciprocating internal combustion engine (i.e., a standard Otto-cycle engine), intake valve closure timing IVC is often fixed to approximately 40 degrees of crankangle ABDC. In the early intake-valve-closing combustion cycle, the magnitude of working angle of an intake valve tends to become insufficient due to such earlier valve closure timing and thus there is a demerit such as the occurrence of abnormal combustion (knocking) as well as reduced engine power output. Hence, in recent years, the late intake-valve-closing combustion cycle is widely adopted to automotive vehicles rather than the early intake-valve-closing combustion cycle. In the case of the late intake-valve-closing combustion cycle, a sufficient working angle can be obtained, but there is an increased tendency for a valve lift to become large too much during low-speed and low-load operation, thereby resulting in an increase in friction loss.

For the reasons discussed above, the specific cam profile of rockable cam 10 of the valve actuation apparatus of the embodiment is configured to ensure the large working angle (the maximum working angle D3), while appropriately suppressing the valve-lift amount, for instance, during low-speed and low-load operation. By this, intake valve closure timing IVC can be controlled to a proper timing value after the piston BDC position on the intake stroke, thus avoiding or suppressing undesirable knocking. Also, by virtue of the appropriately suppressed valve-lift amount (see the top lift L3 of the embodiment suppressed lower than the top lift L3′ of the comparative example in FIG. 9C), it is possible to reduce sufficiently the contact pressure P between cam contour surface 14 of rockable cam 10 and lifter-crown contact surface 9 a of valve lifter 9 even during the maximum valve-lift control mode, thus effectively suppressing an increase in mechanical friction between valve lifter 9 and rockable cam 10.

Furthermore, according to the specific cam profile of rockable cam 10 of the valve actuation apparatus of the embodiment, the radius of curvature of the cam-nose side of rockable cam 10, exactly, the radius of curvature ρ2 of valve-opening middle lift surface 14 d and the radius of curvature ρ3 of valve-opening large lift surface 14 e are set or designed to be less than those of the rockable cam of the comparative example. This contributes to the light-weight rockable cam, consequently, reduced inertia mass, thereby enhancing the responsiveness of operation of the valve actuation system and reduced noise and vibrations.

Moreover, according to the specific cam profile of rockable cam 10 of the valve actuation apparatus of the embodiment, the peak lift L3 (the maximum valve-lift amount) realized by rockable cam 10 of the embodiment can be lowered by a lift difference β in comparison with the peak lift L3′ of the comparative example (see FIG. 9C). The appropriately suppressed valve lift L3 is a properly tuned valve-lift amount suited to produce a satisfactory engine power output, for instance, during high-speed and high-load operation, without sacrificing the engine performance. On the contrary, by virtue of such an appropriately suppressed valve lift L3, in other words, by suppressing an excessive valve-lift amount, it is possible to ensure a sufficient noise/vibration reduction effect, because of reduced friction between valve lifter 9 and rockable cam 10, and reduced fuel consumption rate.

It will be understood that the invention is not limited to the particular embodiments shown and described herein. For instance, the radius of curvature ρ2 of valve-opening middle lift surface 14 d and the radius of curvature ρ3 of valve-opening large lift surface 14 e of cam contour surface 14 of rockable cam 10 may be arbitrarily varied depending on the size, type, and specification of the engine.

In the shown embodiment, the valve actuation apparatus of the embodiment is applied to a multinodular-link, rockable-cam operated valve operating system on the intake valve side. In lieu thereof, the valve actuation apparatus of the embodiment may be applied to a valve operating system on the exhaust valve side.

In the shown embodiment, the valve actuation apparatus of the embodiment is applied to a multinodular-link, rockable-cam operated valve operating system equipped with a variator (a motion converter) 4, configured to simultaneously vary both a valve lift and a working angle. In lieu thereof, the fundamental concept (i.e., the specific cam profile) of the valve actuation apparatus of the invention may be applied to a different type of valve operating device, such as a non-variator equipped valve operating device (a standard valve operating device).

Further Effects (Technical Ideas) of Embodiment

The valve actuation apparatus of the shown embodiments can provide the following further effects (a)-(f).

(a) In a valve actuation apparatus of an internal combustion engine employing drive cam 8, motion-transmission mechanism (15, 16, 17), and rockable cam 10, the radius of curvature ρ3 of valve-opening large lift area 14 e is set to be less than the radius of curvature ρ1 of valve-opening small lift area 14 c, that is, ρ3<ρ1.

(b) The radius of curvature ρ2 of a part of cam contour surface 14 at which an acceleration of the oscillating motion of rockable cam 10 shifts from a positive acceleration area to a negative acceleration area, is set to be less than the radius of curvature Rc of base-circle area 14 a, that is, ρ2<Rc.

(c) In a valve actuation apparatus of an internal combustion engine employing drive cam 8, motion-transmission mechanism (15, 16, 17), rockable cam 10, variator 5, actuator (6, 24), and controller 27, the low-speed and low-load operation comprises idling operation.

Hence, according to the valve actuation apparatus of the embodiment, intake valve closure timing IVC can be controlled to a later timing value after the piston BDC position on the intake stroke, and also it is possible to reduce sufficiently the contact pressure P between cam contour surface 14 of rockable cam 10 and lifter-crown contact surface 9 a of valve lifter 9, thus effectively suppressing an increase in mechanical friction between valve lifter 9 and rockable cam 10.

(d) In a valve actuation apparatus of an internal combustion engine employing drive cam 8, motion-transmission mechanism (15, 16, 17), and rockable cam 10, or in a valve actuation apparatus of an internal combustion engine employing drive cam 8, motion-transmission mechanism (15, 16, 17), rockable cam 10, variator 5, actuator (6, 24), and controller 27, the valve-opening small lift area 14 c is a positive acceleration area extending from ramp area 14 b adjacent to base-circle area 14 a, whereas the valve-opening middle lift area 14 d and the valve-opening large lift area 14 e are a negative acceleration area after a last part of the positive acceleration area has passed the lifter-crown contact surface.

(e) In a rockable cam for use with a valve actuation apparatus of an internal combustion engine, the radius of curvature ρ3 of valve-opening large lift area 14 e, extending continuously from valve-opening middle lift area 14 d toward the cam-nose top, is set to be greater than the radius of curvature ρ2 of a part of valve-opening middle lift area 14 c, bordering valve-opening small lift area 14 c, that is, ρ3>ρ2.

(f) The valve-opening small lift area 14 c is a positive acceleration area extending from a ramp area 14 b adjacent to the base-circle area 14 a, whereas the valve-opening middle lift area 14 d and the valve-opening large lift area 14 e are a negative acceleration area after a last part of the positive acceleration area has passed the lifter-crown contact surface. Additionally, the radius of curvature ρ3 of valve-opening large lift area 14 e of the negative acceleration area is set to be less than the radius of curvature Rc of base-circle area 14 a, that is, ρ3<Rc.

The entire contents of Japanese Patent Application No. 2010-264898 (filed Nov. 29, 2010) are incorporated herein by reference.

While the foregoing is a description of the preferred embodiments carried out the invention, it will be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the scope or spirit of this invention as defined by the following claims. 

1. A valve actuation apparatus of an internal combustion engine comprising: a drive cam adapted to be linked to a crankshaft of the engine in a manner so as to be driven by a transmitted torque from the crankshaft; a motion-transmission mechanism for converting a rotary motion of the drive cam into an oscillating motion; and a rockable cam configured to move with an oscillating motion in synchronism with the oscillating motion produced by the motion-transmission mechanism and having a curved cam contour surface, for opening and closing an engine valve by the oscillating motion of the rockable cam, wherein the cam contour surface of the rockable cam is contoured to have a valve-opening small lift area through which the cam contour surface extends from a base-circle area on which a lifter-crown contact surface rides when the engine valve is closed toward a cam-nose portion, a valve-opening middle lift area extending continuously from the valve-opening small lift area toward the cam-nose portion, and a valve-opening large lift area extending continuously from the valve-opening middle lift area toward a cam-nose top of the cam-nose portion, and wherein a radius of curvature of a part of the valve-opening middle lift area, bordering the valve-opening small lift area, is set to be less than a radius of curvature of the valve-opening large lift area.
 2. The valve actuation apparatus as claimed in claim 1, wherein: the radius of curvature of the valve-opening large lift area is set to be less than a radius of curvature of the valve-opening small lift area.
 3. The valve actuation apparatus as claimed in claim 1, wherein: a radius of curvature of a part of the cam contour surface at which an acceleration of the oscillating motion of the rockable cam shifts from a positive acceleration area to a negative acceleration area, is set to be less than a radius of curvature of the base-circle area.
 4. The valve actuation apparatus as claimed in claim 1, wherein: the valve-opening small lift area is a positive acceleration area extending from a ramp area adjacent to the base-circle area; and the valve-opening middle lift area and the valve-opening large lift area are a negative acceleration area after a last part of the positive acceleration area has passed the lifter-crown contact surface.
 5. A valve actuation apparatus of an internal combustion engine comprising: a drive cam adapted to be linked to a crankshaft of the engine in a manner so as to be driven by a transmitted torque from the crankshaft; a motion-transmission mechanism for converting a rotary motion of the drive cam into an oscillating motion; a rockable cam configured to move with an oscillating motion in synchronism with the oscillating motion produced by the motion-transmission mechanism and having a cam contour surface, for opening and closing an engine valve by the oscillating motion of the rockable cam; a variator configured to vary a valve-lift amount of the engine valve by changing an attitude of the motion-transmission mechanism and consequently by changing a state of the oscillating motion of the rockable cam; an actuator for driving the variator; and a controller configured to control the actuator depending on an operating condition of the engine, wherein the controller is configured to output a control signal to the actuator for bringing an operating characteristic of the engine valve closer to a maximum valve lift and maximum working angle characteristic via the variator during low-speed and low-load operation of the engine, wherein the cam contour surface of the rockable cam is contoured to have a valve-opening small lift surface through which the cam contour surface extends from a base-circle surface on which a lifter-crown contact surface rides when the engine valve is closed via a ramp surface toward a cam-nose portion, a valve-opening middle lift surface extending continuously from the valve-opening small lift surface toward the cam-nose portion, and a valve-opening large lift surface extending continuously from the valve-opening middle lift surface toward a cam-nose top of the cam-nose portion, and wherein a radius of curvature of a part of the valve-opening middle lift surface, bordering the valve-opening small lift surface, is set to be less than a radius of curvature of the valve-opening large lift surface, and the radius of curvature of the valve-opening large lift surface is set to be less than a radius of curvature of the valve-opening small lift surface.
 6. The valve actuation apparatus as claimed in claim 5, wherein: the low-speed and low-load operation comprises idling operation.
 7. The valve actuation apparatus as claimed in claim 5, wherein: the valve-opening small lift surface is a positive acceleration area extending from the ramp surface adjacent to the base-circle surface; and the valve-opening middle lift surface and the valve-opening large lift surface are a negative acceleration area after a last part of the positive acceleration area has passed the lifter-crown contact surface.
 8. For use with a valve actuation apparatus of an internal combustion engine, a rockable cam comprising: a cam contour surface contoured to open and close an engine valve by an oscillating motion of the rockable cam, wherein the cam contour surface is contoured to have a valve-opening small lift area through which the cam contour surface extends from a base-circle area on which a lifter-crown contact surface rides when the engine valve is closed toward a cam-nose portion, a valve-opening middle lift area extending continuously from the valve-opening small lift area toward the cam-nose portion, and a valve-opening large lift area extending continuously from the valve-opening middle lift area toward a cam-nose top of the cam-nose portion, and wherein a radius of curvature of a part of the valve-opening middle lift area, bordering the valve-opening small lift area, is set to be less than a radius of curvature of the valve-opening large lift area.
 9. The rockable cam as claimed in claim 8, wherein: a radius of curvature of the valve-opening large lift area, extending continuously from the valve-opening middle lift area toward the cam-nose top, is set to be greater than the radius of curvature of a part of the valve-opening middle lift area, bordering the valve-opening small lift area.
 10. The rockable cam as claimed in claim 8, wherein: the valve-opening small lift area is a positive acceleration area extending from a ramp area adjacent to the base-circle area; the valve-opening middle lift area and the valve-opening large lift area are a negative acceleration area after a last part of the positive acceleration area has passed the lifter-crown contact surface; and the radius of curvature of the valve-opening large lift area of the negative acceleration area is set to be less than a radius of curvature of the base-circle area. 